1. Field of The Invention
The present invention relates to selective electroless deposition of a metal on a substrate, and more particularly to applying a protective layer on the dielectric so that during electroless deposition the metal is plated only on a catalyst film laser-patterned in a predetermined pattern on the protective layer whether or not the dielectric is catalytic outside the predeterimined pattern.
2. Description of the Related Art
Laser assisted electroless plating has been discussed in the literature in which a laser beam is used to enhance the plating reactions by creating very localized heating. These methods generally suffer the drawback that the horizontal scan rate of the laser is constrained by the slow vertical electroless deposition rates. These slow vertical deposition rates are inherent in the physics of the process and are not likely to be increased substantially. This time delay may make direct laser writing of plated metal impractical when hundreds or thousands of interconnections must be formed on a substrate.
Another technique for patterning metal on a substrate is by laser assisted chemical vapor deposition of a palladium catalyst, as disclosed in U.S. Pat. No. 4,574,095 to Baum. This method, however, uses a temperature controlled vacuum system during laser writing which requires a much more complex and expensive process than wet chemistry.
It is also well known that a polymer film which does not initially contain palladium can be patterned, and palladium can be subsequently applied to the patterned film to form a catalyst for electroless plating, as described in U.S. Pat. No. 4,666,735 to Hoover and U.S. Pat. No. 4,701,351 to Jackson. In both of these methods, however, conventional photolithography techniques are used to pattern the polymer film; that is, a photoimagible polymer is exposed to light through a mask to create the desired pattern. In the case of Jackson this photoimagible polymer is separate from the polymer which complexes the palladium. Finally, in a paper by Cole et al. presented at the
Electrochemical Society Meeting in October, 1987 a method is proposed in which palladium compounds are irradiated with a laser to selectively physically alter the palladium in such a way that only the irradiated areas are catalytic to electroless plating. The palladium acetate disclosed in Cole is not, however, a polymer with complexed palladium ions, but rather an organic metallic compound which is likely to have relatively low solubility in water and
require an organic solvent to achieve satisfactory solubility. Such organic solvent is often chloroform, a carcinogen. In addition, the film must cover the entire substrate prior to laser writing and therefore most of the palladium, an expensive material, may be wasted since in some applications only a small percentage of the substrate area will ultimately contain metal features.
An improved technique for laser-patterned selective electroless plating is described by Hirsch et al. in U.S. Pat. No. 4,981,715 and U.S. application Ser. No. 07/600,361 filed Oct. 19, 1991 which are assigned to the assignee of the present invention and incorporated herein by reference. In one embodiment a substrate is coated with a solution of polymer-seed metal mixture which can initiate electroless plating, the mixture is dried and selectively irradiated in a predetermined pattern, the non-irradiated mixture is etched off, and the substrate is placed in an electroless bath which deposits metal on the substrate in the predetermined pattern. In another embodiment a substrate is immersed in a polymer solution, polymer is deposited where the substrate is irradiated in a predetermined pattern, the substrate is removed from the polymer solution, the deposited polymer is contacted with a solution of seed metal ions which can initiate electroless plating to form a complexed deposited polymer, and the substrate is placed in an electroless bath which deposits metal on the complexed deposited polymer in the predetermined pattern.
Nevertheless, Applicant has observed an electroless "sheeting" problem in which widespread deposition of metal occurs in unwanted catalytic areas on the dielectric (i.e., those not irradiated by the laser) during the electroless plating step. The ability of a polyimide surface to catalyze electroless metal may result inadvertently from various substrate fabrication processes. For instance, depositing metals on polyimide by either sputtering or evaporation may create a conductive carbonized surface film. Also, improper plasma etching can create an undesired surface film such as teflon. Polyimide modified to contain such films may become catalytic and initiate undesired electroless deposition. Furthermore, the observed sheeting problem has been random and difficult to predict in advance for a particular substrate.
Needless to say, there is a need for preventing unwanted electroless sheeting on a dielectric which has inadvertently become catalytic in certain regions.